In general, as a method for measuring film thickness—in particular, for measuring film thickness of multi-layer optical thin film, ellipsometry method is known, which is described in the Non-Patented Reference 1 given below. According to this method, a light is projected to a surface to be measured by changing polarizing status of the incident light. Then, the polarizing condition of the reflection light is determined and by performing parameter fitting of the measured data, optical constant and thickness of the thin film can be determined. This ellipsometry method is widely used in the inspection device for the thin film forming procedure in the semiconductor device manufacturing process.
In particular, the size of the substrate (normally, glass substrate) to be used in a flat type image display device (the so-called “flat panel display” (FPD); hereinafter, also called “display”) represented by a liquid crystal panel is getting larger and larger in recent years. When a CVD device normally used for deposition of thin film is used to form thin film on the glass substrate, in-plane film thickness variation (amount of change in film thickness distribution on portions of glass substrate with different film deposition surface; variation of in-plane film thickness of thin film) is increased when the size of the glass substrate becomes larger. For this reason, it is necessary to have as many measuring points as possible on the surface of the glass substrate and to control the range of variation in film thickness on the surface of the glass substrate by measuring detailed distribution of the in-plane film thickness variation.
This is because, when the thin film deposited on the glass substrate is a silicon film, the in-plane film thickness variation may exert serious influence on the characteristics of semiconductor device to be formed on the silicon film. As an example, description will be given below on a manufacturing line of low temperature silicon thin film transistor (TFT) substrate (hereinafter, also called “TFT substrate”) used in FPD.
The TFT substrate as described above comprises a pixel circuit and a driving circuit incorporated in silicon semiconductor thin film (hereinafter simply called “semiconductor”) deposited on insulator substrate (hereinafter simply referred as “substrate”). Also, as driving elements to constitute the pixel circuit or the driving circuit, thin film transistor (TFT) is used in many cases. By using a polycrystal semiconductor thin film (typically called “polysilicon film or poly-Si film) instead of an amorphous semiconductor thin film (typically called “amorphous silicon film” or referred as “a-Si film”) as active layer of the thin film transistor, it is possible to achieve image display with high precision and with high image quality.
The reason for this may be that the polycrystal silicon semiconductor thin film has higher mobility of carrier (electron in n-channel; hole in p-channel) compared with the amorphous silicon semiconductor thin film. As a result, high precision can be attained by reducing cell size (pixel size). For the formation of the thin film transistor deposited on normal poly-silicon semiconductor thin film, high temperature of 1000° C. or higher is required. On the other hand, in the technique to form poly-silicon semiconductor thin film at a low temperature on silicon layer by laser annealing, the substrate with semiconductor thin film deposited on it is not heated to high temperature, and it is possible to form the thin film transistor (TFT) with high mobility in low temperature process, and a low cost glass substrate can be used.
The effect that the mobility is high is advantageous in that the size of TFT can be reduced. This advantage leads to high brightness attained by the increase of opening ratio in the area of the openings other than TFT structure, comprising pixel circuits within pixel region for FPD, and this also results in the improvement of precise image quality. In the low temperature polysilicon TFT manufacturing process, the process influenced by the variation in film thickness of the amorphous silicon film is the annealing process using laser.
For example, in case annealing is performed by a pulsed excimer laser, and when it is checked how the grain size (average grain size) of polycrystal silicon to be crystallized is changed due to laser energy density, it is found that grain size of polycrystal silicon tends to be larger when laser energy is increased. However, when energy exceeds a certain energy threshold (when it is on the excess side), it is turned to microcrystal. This will be described later by referring to FIG. 7.
The dependency of polycrystal silicon grain size on laser energy is changed due to film thickness of the silicon thin film. If the film is thick, laser energy is turned to shortage in proportion to the increase of film thickness. If the film is thin, laser energy is turned to be in excess. Therefore, film thickness distribution causes the difference in the in-plane distribution of grain size of polycrystal silicon and also leads to the difference in average particle size for each substrate. Finally, it causes the variations of TFT characteristics. In “the Patented Reference 1”, it is described that laser annealing is performed on the amorphous silicon substrate before laser annealing through adjustment of the energy necessary for reforming of the thin film by amorphous silicon film thickness measured by ellipsometry only for the first substrate in mass production lot.
Next, description will be given on a method for measuring film thickness other than the method based on ellipsometry. In “the Non-Patented Reference 2”, a method for measuring film thickness called “R, T, t method” is described. According to this method, optical constant of thin film of single layer is determined by film thickness data measured by transmissivity and reflectivity of a vertical incident light, and this is not a method for determining film thickness. “The Patented Reference 2” describes a method for evaluating film thickness by projecting a light of multiple wavelengths and by measuring reflection spectra. This is also a method for determining film thickness by parameter fitting. “The Patented Reference 3” discloses a method called “BPR method”. This is a method to evaluate film thickness through measurement of the dependency of reflection light intensity on reflection angle by using a projection system with multiple incident angles. This is also a method to determine film thickness by parameter fitting.
[Non-Patented Reference 1] JOSA, Volume 58 (1968), p. 526.
[Non-Patented Reference 2] Applied Optics 23 (1984), pp. 3571-3596.
[Patented Reference 1] JP-A-2003-258349
[Patented Reference 2] JP-A-2002-81916
[Patented Reference 3] Japanese Patent Publication No. 3337252
[Patented Reference 4] JP-A-2003-109902